Limit switch and control system combination



Sept. 7, 1943. P. s. WHITE 2,328,697

LIMIT SWITCH AND CONTROL SYSTEM COMBINATION- Filed Sept. 5, 1942 -36 3g32 FIG-1 26 b a l I 34 2 1 22 36 a0 {I p 35 a4 2o 24 a4 FIG-2 52INVENTOR. PAUL G- Wane.

Patented Sept. 7, 1943 LIMIT SWITCH AND CONTROL SYSTEM COMBINATION PaulG. White, Cleveland Heights, Ohio, assignor to The Electric Controller &Manufacturing Coiinpany, Cleveland, Ohio, a corporation 0 0 ApplicationSeptember 5, 1942, Serial No. 457,464

9 Claims.

This invention relates to the control of electric motors by limitswitches and particularly to control systems for apparatus wherein theextent of operation of one motor driven element must be limited withreference to the position of another element which is independentlyoperated by another motor. The invention is. particularly suitable forcontrolling electrically operated cranes of the boom type, and ishereinafter described as applied to that use.

Electrically operated boom cranes-are generally operated by twoindependent motors. One of the motors drives the winding drum for theboom hoist cable, and the other motor drives the winding drum for theload or hook hoist cable. The boom hoist cable controls the raising andlowering of the boom and extends from'the boom hoist winding drum overan idler pulley, mounted above the front end of the crane cab, to itspoint of attachment near the free end of the boom. The other or fixedend of the boom is pivoted at the bottom of the front end of the cranecab. The boom thus may swing in a vertical plane about its pivot as theboom hoist cable is wound on or unwound from the boom hoist windingdrum. The load or hook hoist cable extends from the load or hook joistwinding drum over an idler pulley at the free end of the boom andextends downward therefrom to a hook or other suitable device to which aload may be attached.

The crane cab is pivotally supported at a point near its center so as tobe rotatable in a horizontal plane. To counterbalance the weight of theboom and the force of the load which is transmitted to the crane cab atthe pivot point of the boom, the driving motors and winding drumsgenerally are mounted at the rear of the crane cab.

Because of this location of the motors and drums and the arrangement ofthe boom hoist cable, hook hoist cable, and boom, each of the cables andthe boom swing about a different pivot upon raising and lowering of theboom and consequently a difierential in movement for any given point onthe boom hoist cable, the hook hoist cable, and the boom results. Forexample, if the boom is at its uppermost limit and the hook hoist cableis operated so that the hook is slightly below the end of the boom,lowering of the boom causes a take-up in the length of the hook hoistcable between the boom pulley and the hook and jams the hook into theboom pulley. On the other hand, it the boom is horizontal and the hookis drawn to within the usual close distance from the boom pulley and theboom is then raised,

there is an excess of hook hoist cable in the raised position of theboom so that the hook is disposed several feet below the end of theboom. If the boom hoist motor has operated to lower the boom to itslowermost position, and the load hoist motor is operating to raise thehook ,automatic means should be provided to stop the load hoist motorbefore the hook reaches the pulley at the end of the boom. However, ifthe hook is stopped by the automatic means at its upper limit of travelwhen the boom is in its lowermost position, raising of the boom withoutoperating the main hoist motor allows the hook to drop farther from thepulley at the end of the boom since the distance from the hook hoistdrum to the boom pulley becomes less. Heretofore in the use of such limt switches, a choice had to be made between the danger of jamming thehook into the boom pulley upon lowering of the boom or limiting themaximum height 01' lift.

Because of these conflicting factors, an ordinary limit switch driven inthe usual manner by the load hoist drum and operable automatically tostop the load hoist motor at the limit of travel of the hook when theboom is in its lowermost position does not permit the hook to rise toits full limit of travel when the boom is in a. raised position. On theother hand, an ordinary limit switch driven in the usual manner by theload hoist drum and adjusted to stop the book just before it reaches theboom pulley when the boom is in a raised position, does not provideprotection against jamming by hoisting of the hook when the boom is in alower position.

One of the objects of the present invention is to eliminate both ofthese difliculties and make it possible to operate a boom crane withoutdanger or jamming the hook and the boom pulley regardless of theposition of the boom and without limiting the maximum lifting height ofthe crane in anyjnanner other than by the length of the boom itself.

In accordance with the present invention, an ordinary and common limitswitch of the rotatable type is driven through a differential gear byboth the boom hoist and the hook hoist drums. The apparatus is soarranged that the limit switch is driven in one direction duringhoisting of the hook or lowering of the boom and is driven in theopposite direction during lowering of the hook or raising of the boom.Consequently, the operation of the limit switch is compensated for boomposition. A limit switch means is thus provided which permits the hookto approach as closely to the boom pulley as is safe. but which hascontacts which stop the load hoist motor when the hook approaches tooclosel to the boom regardless of the position of the boom and which stopthe boom hoist motor when the boom is lowered to a position where theboom pulley is approaching too close to the hook. The limit switchcontacts also prevent raising of the hook or lowering of the boom whilethe hook and boom pulley are too close together.

An object of this invention is to provide a simplifled limit switchcontrol for controlling the operation of independently driven elementsin accordance with their relative positions.

Another object is to provid a differentially operated limit switch meansfor preventing independently driven element from approaching too closeto each other.

A further object is to provide a driving system for a rotatably drivenlimit switch of the usual and common type so that the limit switch isoperable to stop independently driven elements in predetermined relationto each other.

A more specific object is to provide an improved limit switch controlsystem for a boom crane.

Another specific object is to provide an overhoist limit switch controlsystem for a boom crane which is compensated for boom position.

A further specific object is to provide a limit switch control systemfor a boom crane which effects stopping of the boom hoist motor if theboom is lowered within a predetermined distance from the hook and which.regardless of the position of the boom, effects stopping of the hookhoist motor if the hook is raised within said predetermined distancefrom the boom.

Other objects and advantages will become ap parent from the followingspecification wherein reference is made to the drawing, in which Fig. 1is a diagrammatic representation of a boom crane of the type to whichthis invention may be applied, and

Fig. 2 is a very much simplified wiring diagram and diagrammaticrepresentation of the appa ratus of this invention.

Referring to Fig. l, a cab ill of a boom crane I2 is provided with adownwardly extending pivt pin i4 and suitable rollers i6 so that the cabIll can be centrally supported for rotation in a horizontal plane abouta point near its center. A boom Ill has its fixed end pivoted at Illnear the lower front or right hand end of the cab in so that the boom IImay be raised and lowered in a vertical plane and rotation of the cabIll about the support for the pivot pin i4 may swing the boom 08horizontally;

A boom hoist cable 22 for controlling the raising and lowering of theboom l8 has one end attached near the free end of the boom [8 at 24 andextends to a boom hoist winding drum 30 over an idler pulley 26supported by a framework 28 extending upwardly from the front end of thecab l0. boom winding drum an which ismounted in the rear or left handend of the cab iii. A load or book hoist cable 32 has a suitable loadattaching device such as a hook 44 fastened to one end and passes over aboom pulley 86 at the free end of the boom 88 to a. load or book hoistwinding drum 88 mounted in front of the drum 34 near the rear end of thecab Ill. The winding drums 30 and 38 and the driving mo-L tors thereforare mounted in the rear end of the cab ill to act as a counterbalancefor the weight The cable 2-2 is wound on the oi. the boom is and theload being lifted. The boom l8 and the cables 22 and 32 are shown insolid lines to indicate their respective positions when the boom I8 isin the lowermost position and are shown in broken lines to indicatetheir respective positions when the boom I8 is in the uppermostposition.

Referring to Fig. 2, the hook hoist drum 3B is arranged to be driventhrough suitable gearing 40 by a direct current motor 42 having asuitable energized field winding 44. The boom hoist drum 30 is arrangedto be driven through suitable gearing 46 by a direct current motor 43having a suitable energized field winding 50. Reversible power for thearmature winding of the motor 42 is obtained from a suitable directcurrent source 52 through electromagnetic reversing switches 54 and 56.Reversible power for the armature winding of the motor 48 is obtainedfrom the source 52 through electromagnetic reversing switches 58 and 60.The operation of th switches 54 and 56 is controlled by a mastercontroller 62 and the operation of the switches 58 and 60 is controlledby a master controller 64. The controllers B2 and B4 are shown in theiroff positions. Power for operating the switches 54, 5 8, 58, and 60 isobtained from the source 52. Although direct current motors have beenillustrated, it is obvious that the invention may be used withalternating current motors as well.

Movement of the controller 62 to the right from its off positionnormally energizes the switch 54 which when energized operates toconnect the motor 42 to the source 52 from driving the hook hoistdrum 3Bin the direction causing hoisting of the hook 214, and movement of thecontroller 62 to the left from its off position energizes the switch 56which when energized operates to connect the motor 42 to the source 52for driving the hook hoist drum 38 in the direction causing lowering ofthe book 34. Likewise, movement of the controller 64 to the right fromits off position normally energizes the switch 58 which when energizedoperates to connect the motor 48 to the source 52 for driving the boomhoist druni 30in the direction causing raising of the boom I8. andmovement of the controller 64 to the left from its off position normallyenergizes th switch Gil which when energized operates to connect themotor 48 to the source 52 for driving the boom hoist drum 30 in thedirection causing lowering of the boom I8. I

The switch 56 may be energized. at any time upon movement of thecontroller 52 to the left, but the switches 54, 58, and Eli areenergized upon movement of their associated controllers 62 or 64 only ifcertain limit switches now to be described are in the proper positions.The energizing circuit for the switch 54 can be completed only whencontacts 58 of a suitable rotatable type limit switch, such as a camlimit switch 68, are

closed. The limit switch 68 also comprises a rotatable circular cam Illhaving a projecting portion 12 which cooperates with a pusher rod 14 toopen and close the contacts 86 depending upon the rotated posltionot thecam 10. The energizlng circuit for the switch 58 can only be completedwhen limit switch contacts 15 are closed. The limit switch contacts 15are closed when the boom i8 is below its upper safe limit of travel andare open only when the boom Ill is at its upper safe limit of travel.The energizing circuit for the switch 60 can only be completed whencontacts Hi and B6 are closed. The contacts 16 are closed when the boom88 is above its lowermost sai'e limit 01' travel and are open only whenthe boom I3 is at its lower sate limit of travel. The contacts 15 and 16may be parts or a hatchway type limit switch such as i commonly used onboom cranes to limit the extent or travel of the boom I 8 independentlyof the position of the hook 34.

The cam 10 is arranged to be rotatably driven by a driven shaft 18 o! astandard differential gear 83. One driving element of the differentialII) is driven by the hook hoist drum 38 through bevel gears 32 andgearing 84. The other driving element or the differential 80 is drivenby the boom hoist drum 38 through bevel gears 83 and reversing gearing38.

When the hook 34 is as close to the boom pulley 36 as possible formaximum safe height of lift and the boom I3 is in its lowermostposition, the cam I0 is adjusted on the shaft 13 so that the raisedportion 12 engages the rod 14 to hold the contacts 56 open, as shown inFig. 2. With the limit switch 58 in the open position the motor 42cannot be energized to raise the hook 34 and the motor 48 cannot beenergized to lower the boom l8 due to the open contacts 88 in theenergizing circuits for the switches 54 and 6B.

Assume that the boom I8 is in its lowermost position and that the hook34 has been raised to its upper limit leaving the length a (Fig. 1) ofthe cable 32 between the hook 34 and the pulley 35. Movement of thecontroller 64 to the left does not cause energization of the switch 60because the contacts 65 and 15 are open, and movement of the controller62 to the right does not cause energization of the switch 54 because thecontacts 66 are open. Movement of the controller 84 to the right,however, causes energization of the switch 58 which operates to causethe motor 48 to drive the boom hoist drum in the direction which raisesthe boom l8. Movement of the boom H! from it lowermost position effectsclosure of the contacts 16 in a well known manner. Rotation of the drum3!! in the direction which raises the boom l8 drives the differential 80in a direction to cause the cam ill to rotate counterclockwise, and whenthe portion 12 passes from beneath the rod '14, the contacts 65 close.When the boom l8 reaches its upper limit as shown by the dotted lines inFig. l, the contacts 15 open to deenergize the switch 58 which opens tostop the boom hoist motor 48. Due to the geometric configuration of thecables 22 and 32 and the boom l8. the length of the cable 32 between thehook 34 and the pulley 36 has increased from the length a to the lengthindicated by b as a result of raising the boom l8. However. closure ofthe contacts 66 upon raising of the boom l8 permits further hoisting ofthe hook 34 so that the increase in the length of the cable 32 betweenthe boom pulley 38 and the hook 34 may be taken up by further hoistingof the hook 34.

For example, at any time after the contacts 66 close, movement of thecontroller 62 to the right causes operation of the switch 54 to encrgizethe motor 42 to drive the drum 32 in a direction to hoist the hook 34.Rotation of the drum 32 in a. direction to hoist the hook 34 drives thecam 10 through the differential 8B in a clock- Wise direction, and, ifthe book 34 overtakes the boom IS, the projecting portion 12 of the camHi again strikes th rod 14 to open the contacts 55 to stop the motor 42and prevent the starting of the motor 43. The hook 34 is thusautomatically prevented from approaching within a predetermined distancefrom the boom pulley regardless of the position or the boom l8, and maybe hoisted to its maximum sate height while the boom I4 is in a raisedposition.

At any time that the hook 34 and boom pulley 34 are far enough apart forthe contacts 63 to be closed and the boom i8 is not in its lowermostposition determined by the contacts 16, movement of the controller 54 tothe left causes energization of the switch 5|] which when enersizedcloses to cause the boom hoist motor 48 to drive the boom hoist drum 3!!in a direction to lower the boom l3, and the cam 10 is driven in aclockwise direction. The boom l8 may continue to lower until the boompulley 3B nears the hook 34 at which time the projecting portion 12 ofthe cam Ill lifts the rod 14 to open the contacts 33 or until the boomI8 reaches its lowermost position and opens the contacts 16.

Likewise, if the hook 34 and boom pulley 35 are far enough apart for thecontacts 56 to be closed, movement of the controller 52 to the rightcauses energization of the switch 54 which operates to causeenergization of the motor 42 so that the hook hoist drum 33 is driven ina direction to hoist the hook 34. The drum 3!! when hoisting the book 34drives the cam Ill in a clockwise direction to move the projectingportion 12 toward the rod I4. When the hook 34 again approaches the boompulley 36, the projecting portion 12 moves the rod 14 upward to open thecontacts 66. Opening of the contacts 66 deenergizes the switch 54 whichthereupon opens to deenergize the motor 42 and prevents the reclosure ofthe switch 60 so that the boom l8 cannot be lowered.

The hook 34 may be lowered at any time regardless oi the position of theboom l8 by moving the controller 62 to the right to energize the switch56 which, when energized, connects the motor 42 so as to drive the drum38 in a direction to lower the hook 34. When rotating in this direction,the drum 3!) drives the cam 10 through the differential in acounterclockwise direction so that the projection portion 12 moves tothe left away from the rod 14. There is no limit to the extent oflowering of the hook provided by the limit switch 68 since it is assumedthat all of the main hoist cable will be paid out before the cam TB hasmade a complete revolution.

The diiierential connection between the drums 33 and 38 and the limitswitch 68 ,compensates the, limit switch 68 for the position of the boom[8. The accuracy of the limit switch compensation provided dependssomewhat upon the geometric configuration of the pivot point 20, pointof cable attachment 24 on the boom l8, pulleys 36 and 26, and the drums30 and 38. Suitable gear ratios may be selected toinsure that theaccuray of compensation is adequate for any particular installation.

The control system for the motors 42 and 48 has been shown greatlysimplified for ease of understanding of the present invention. It isunderstood that actual control systems would be much more complicatedincluding among other things acceleration resistance control and dynamicbraking or other electrical braking connections.

I claim:

1. In a control system for separately operable reversible motorsarranged to drive separately operable elements, respectively, adifl'erentially operated limit switch means for the motors and drivendifferentially in response to the operated positions of said operableelements, said limit erable elements, said limit switch means havingcontacts operable for controlling the energize tion of said motors whensaid motors are driving said operable elements toward each other.

3. A control system for separately operable reversible motors arrangedto drive separately operable elements, respectively, comprisingenergizing circuits for said motors, respectively, a difierential gearhaving driving elements and a common driven element, means operativelyconnecting the driving elements of said differential gear to saidoperable elements, respectively, for driving said driving elements in afixed relation to said operable elements, respectively, a limit switchmeans operatively connected to the driven element of said differentialgear and driven thereby, and said limit switch means being operativelyassociated with said energizing circuits to control limits of relativepositions of said operable elements by control of the energization ofthe motors.

4. A control .system for separately operable motors arranged to driveseparately operable elements, respectively, comprising switching meansfor controlling the energization of said motors, a differential gearhaving driving elements and a common driven element, means operativelyconnecting the driving elements of said differential gear to saidoperable elements, respectively, for

driving said driving elements in a fixed relation to said operableelements, respectively, a limit switch operatively connected to thedriven element of said differential gear and driven thereby, and saidlimit switch being operatively associated with said switching means tocontrol limits of relative positions of said operable elements bycontrol of the energization of the motors.

5. In a boom hoist, a boom, a boom hoist mechanism connected thereto forhoisting and lowering of the boom, an independently operable load hoistdrum, a load hoist cable connected thereto for hoisting and lowering ofa load independently of the boom, independently operable motors for saidboom hoist mechanism and said load. hoist drum, respectively, adifferentially operable limit switch means for the motors and drivendifferentially in response to the operated positions of the boom and theload hoist cable for maintaining predetermined limits of relativepositions of the boom and the load hoist cable.

6. In a boom hoist, a boom hoist drum, a boom hoist cable connectedthereto for hoisting and lowering of the boom, an independently operableload hoist drum and a load hoist cable connected thereto for hoistingand lowering of a load independently of the boom, independently operablemotors for said drums, respectively, and a diilerentially operable limitswitch means for the motors and driven differentially in response to theoperated positions of the boom and load hoist cables for maintainingpredetermined limits of relative positions of the boom and load hoistcables.

'7. In a. boom hoist, a boom hoist drum, a boom hoist cable connectedthereto for hoisting and lowering of the boom, an independently operableload hoist drum, a load hoist cable connected thereto for hoisting andlowering of a load independently of the boom, independently operablemotors for said drums, respectively, a limit switch for controlling theoperation of one of said motors to limit the range of operation of saidboom hoist drum, and a difierentially operable limit switch means forthe motors and driven differentially in response to the operatedpositions of the boom and load hoist cables for maintainingpredetermined limits of relative positions of the boom and load hoistcables.

8. In a boom hoist, a boom, a boom hoist mechanism conected thereto forhoisting and lowering of the boom, an independently operable load hoistdrum, a load hoist cable connected thereto for hoisting and lowering ofa load independently of the boom, independently operable motors for saidboom hoist mechanism and said load hoist drum, respectively, adifferential mechanism having a. plurality of driving elements and acommon driven element, means for driving said driving elements in fixedrelation to the operation of the boom and the load hoist cable,respectively, and limit switch means for the motors driven by saidcommon driven element of said differential mechanism for maintainingpredetermined limits of relative positions of the boom and the loadhoist cables.

9. In a boom hoist, a boom, a boom hoist drum, a boom hoist cableconnected thereto for hoisting and lowering of said boom, 2. loadattaching device, an independently operable load hoist drum, a loadhoist cable connected to said lead hoist drum and said load attachingdevice and associated with said boom for hoisting and lowering of saidload attaching device independently of the boom, independently operablereversible motors for said drums, respectively, a differential mechanismhaving a plurality of driving elements and a common driven element,means for driving said driving elements in fixed relation to theoperation of the boom and load hoist drums, respectively, and limitswitch means for the motors and driven by said common driven element ofsaid differential mechanism, said limit switch means including contactsfor controlling the operation of said motors to maintain a predeterminedminimum distance between said boom and said load attaching deviceregardless of the hoisted position of said boom.

PAUL G. WHITE.

